Active brazes and active brazing methods are used to join components.
An example of the application of this includes, for example, ceramic pressure sensors described in European patent, EP 490 807 A1 and German patent, DE 10 2010 043 119 A1, having                a ceramic measurement membrane that can be supplied with pressure and elastically deformed as a function of the pressure,        a ceramic base body, and        an active-brazed connection that connects an outer edge of a first side of the measurement membrane including a pressure chamber to an outer edge of a face side of the base body facing the measurement membrane.        
To active braze ceramic components, active brazes that have a thermal expansion coefficient that is as similar as possible to that of the ceramic are preferably used. The ternary active brazes described in EP 490 807 A1, which have a Zr—Ni alloy and titanium, are, in this respect, suitable for joining components made of aluminum oxide ceramics. During active brazing, the active component of the active braze—here, titanium—reacts with the ceramic. A mechanically high-strength chemical bond between the ceramic and the active braze is made by reducing the ceramic.
In EP 490 807 A1, an active-brazed connection made using a ring-shaped brazing preform is described, the height of which corresponds to the height of the pressure chamber included under the measurement membrane. Due to the melt-spinning method used to produce them, the brazing preforms described in EP 490 807 A1 have a minimum thickness on the order of 30 μm.
In capacitive ceramic pressure sensors that have at least one capacitor formed by an electrode applied to the measurement membrane and by a counter electrode applied to the face side of the base body facing the measurement membrane and having a capacity dependent upon the pressure-dependent deflection of the measurement membrane, the distance between the electrode and the counter electrode regularly depends upon the thickness of the measurement membrane and the active-brazed connection that bonds the base body.
A lower minimum thickness of the active-brazed connection that bonds the measurement membrane and the base body and, accordingly, lower electrode spacing that is better-suited to achieving higher measurement accuracy can be achieved according to a method described in DE 10 2010 043 119 A1. The active braze required for the active-brazed connection is applied to the joining surface via vapor deposition. In this way, joints having a lower structural height, e.g., with a thickness of 10 μm, can be produced.
Furthermore, an active brazing material is described in the as yet unpublished German patent application 10 2014 113858.5 of the applicant dated Sep. 26, 2014                which consists of layer sequences arranged on top of one another,        the layer sequences of which consist of layers of brazing material arranged on top of one another,                    each of which consists of a component of a base active braze and, in conjunction with each other, contains all components of the base active braze, and            the quantity and layer thickness of which are coordinated with each other in such a way that the components of the base active braze are contained in the active brazing material in a quantity ratio that corresponds to the target composition of the base active braze.                        
In all three cases, the brazing is carried out by heating an arrangement as a whole formed by the base body, the brazing preform, and the measurement membrane to a brazing temperature above the melting point of the braze and keeping it there over an extended period of time—in particular, a period of time between 5 min and 15 min. In conjunction with the aforementioned ternary active brazes having a Zr—Ni alloy and titanium, temperatures above 800°C.—in particular, temperatures in the range of 890°C. to 920°C.—are regularly necessary for this purpose. Active brazing is thus a time- and energy-intensive process.
Because the components to be joined must, as a whole, be brought to this comparatively high temperature above the melting point of the active braze, it takes a correspondingly long amount of time for the joint to cool down after the brazing process. Long cooling times favor the formation of more coarse-grained structures within the joint, due to the precipitation of phases. This may cause inhomogeneities within the joint that impair the strength of the joint.
In U.S. Pat. No. 7,361,412 B2, brazing methods are described in which the heat energy required for the brazing is produced by reactive multilayer systems supplied as a film. Reactive multilayer systems comprise thin layers of reactants, alternately deposited on top of one another, that react with each other exothermally, such as nickel and aluminum. Triggered by local activation of the multilayer system, the reactants form inter-metallic phases. If the enthalpy of formation of the phase formation is high enough, the reaction heat released locally as a result of this causes the exothermic reaction to continue automatically along the multilayer system. This process becomes faster as the individual layers become thinner. Depending upon the thickness of the layers, propagation velocities on the order of 30 m/s are reached. The heat energy released in the process becomes greater as the multilayer system becomes thicker. Temperatures of over 1000°C. are reached. Because the heat energy is supplied only very briefly and very locally, the joint cools down very quickly again after the brazing process. Due to the faster cooling, joints are formed that are more fine-grained and more homogeneous, and thus have more strength.
The brazing methods described in U.S. Pat. No. 7,361,412 B2 all follow the same basic principle, according to which a reactive multilayer system is arranged between two components to be brazed, and at least one layer of brazing material is arranged on both sides of the multilayer system between the multilayer system and the respective component. Thus, an arrangement having the following stacking order is formed: component, braze, multilayer system, braze, and component. The two components are joined by pressurizing this arrangement at room temperature and triggering the reaction of the multilayer system. Multilayer systems having a thickness of 150 μm and more are required to provide the required amount of energy for brazing. As a result, the height of joints produced in this way is correspondingly high, such that these methods for joining the measurement membrane and base body of capacitive ceramic pressure sensors are regularly unsuitable.